1. Field of the Invention
The present invention pertains to the field of microwave polarizers of the type used for selecting a desired signal from among multiple signals of diverse polarity, and more particularly is directed to improvements in a polarizer of the electronic type where polarity selection is achieved by rotation of the incoming signals through a magnetic field.
2. State of the Prior Art
A polarizer is a device which, among other applications, finds widespread use in parabolic dish antenna systems for selecting from among multiple signals transmitted on a common frequency but with different polarization, as is common practice in satellite communications. The polarization selection in such systems can be achieved by either mechanical or electronic means. In a typical mechanical system, a low noise amplifier mounted at the antenna focus is rotated so that its rectangular input waveguide is aligned with the desired signal polarization and rejects signals of different polarization. Mechanical systems are however vulnerable to adverse weather conditions such as icing which can immobilize the device, and also tend to be slow in physically rotating the low noise amplifier by means of a small gear motor.
It is well known that the polarization plane of an electromagnetic signal in a waveguide may be rotated by passing the polarized signal through a magnetic field. This characteristic of polarized signals has been exploited in the past to make so-called electronic polarizers which typically consist of a circular input waveguide capable of accepting signals irrespective of polarization, a rectangular output waveguide which admits signals of a particular polarization while rejecting signals of different polarization, and an intermediate waveguide section surrounded by a coil. A magnetic field of adjustable intensity is created within the intermediate waveguide by a variable current applied to the coil. It is conventional to place ferrite or equivalent material within the intermediate waveguide section to thereby enhance the intensity of the polarization controlling magnetic field.
If no current is applied to the coil, a signal applied to the input of the polarizer passes unaffected through the ferrite to the output waveguide. If a current is applied to the coil, the ferrite becomes magnetized and a signal passing through the resultant magnetic field will have its plane of polarization progressively rotated as it advances through the magnetic field. The current to the coil can be adjusted to vary the strength of the magnetic field, thereby adjusting the total angle of rotation between the input and output polarization planes so as to align the output polarization plane of the desired signal with the rectangular output waveguide. The output waveguide will then pass only the desired signal and reject all others.
For optimum results, the intermediate waveguide should be filled with the ferrite material, which in turn requires a reduction in the waveguide cross-section due the considerably higher dielectric constant of the ferrite material compared to atmospheric air. Furthermore, impedances must be matched both at the input air-ferrite and output ferrite-air transitions, which is accomplished with high dielectric constant ceramic transformers. The input and output ceramic transformers and the ferrite slug must fit together with no air gaps between them. Unless these parts fit together within close tolerances, the intervening air gaps cause poor frequency response and signal drop-out at particular frequencies (known as mode spikes).
Presently, Ku band polarizers of the aforedescribed type require components dimensioned to very close tolerances, e.g. 0.0005 inches, and frequently also some way of tuning the device for optimum frequency response. These requirements result in high manufacturing costs which make such devices unattractive for use in consumer market satellite broadcast reception systems.
Electronic polarizers are more reliable than mechanical systems because of the absence of any moving parts and are therefore unaffected by freezing weather and moisture. Further, the polarization switching action of an electronic polarizer is nearly instantaneous for all practical purposes, as compared to the slow mechanical rotation of a motor driven system.
A continuing need therefore exists for lower cost electronic type polarizers which do not significantly degrade weak satellite transmission signals.